This invention relates to aluminum base alloy products, and more particularly, it relates to an improved lithium-containing aluminum alloy flat rolled product and a method of producing the same.
In the aircraft industry, it has been generally recognized that one of the most effective ways to reduce the weight of an aircraft is to reduce the density of aluminum alloys used in the aircraft construction. For purposes of reducing the alloy density, lithium additions have been made. However, the addition of lithium to aluminum alloys is not without problems. For example, the addition of lithium to aluminum alloys often results in a decrease in ductility and fracture toughness. Where the use is in aircraft parts, it is imperative that the lithium-containing alloy have both improved fracture toughness and strength properties.
It will be appreciated that both high strength and high fracture toughness appear to be quite difficult to obtain when viewed in light of conventional alloys such as AA (Aluminum Association) 2024-T3X and 7050-TX normally used in aircraft applications. For example, a paper by J. T. Staley entitled "Microstructure and Toughness of High-Strength Aluminum Alloys", Properties Related to Fracture Toughness, ASTM STP605, American Society for Testing and Materials, 1976, pp. 71-103, shows generally that for AA2024 sheet, toughness decreases as strength increases. Also, in the same paper, it will be observed that the same is true of AA7050 plate. More desirable alloys would permit increased strength with only minimal or no decrease in toughness or would permit processing steps wherein the toughness was controlled as the strength was increased in order to provide a more desirable combination of strength and toughness. Additionally, in more desirable alloys, the combination of strength and toughness would be attainable in an aluminum-lithium alloy having density reductions in the order of 5 to 15%. Such alloys would find widespread use in the aerospace industry where low weight and high strength and toughness translate to high fuel savings.
When the aluminum-lithium alloy is a flat rolled or sheet product, yet further problems occur. For example, when the sheet product is stretched, it often forms Luder's lines. Lueder's lines are lines or markings appearing on the otherwise smooth surface of metal strained beyond its elastic limit, usually as a result of a multi-directional forming operation and metal movement during that operation.
Luder's lines are objectional from an appearance standpoint. Normally, polishing does not remove the markings resulting from the formation of such lines. If the sheet product is clad, then polishing could be detrimental by making the cladding thickness nonuniform. Also, in a product having the thickness of sheet, too much polishing can affect the mechanical properties. A further problem with formation of Luder's lines is that they often occur nonuniformly. Thus, it will be appreciated that because of these problems, it is desirable to provide sheet product free of Luder's lines.
The present invention provides an improved lithium-containing aluminum base alloy flat rolled product which can be processed to provide a sheet or plate product, for example, which is substantially free of Luder's lines after stretching.